Due to the issues I had with heads, have been doing some research.
From that some things flow to the top.

1. Vendors selling heads for big bucks are lying just as those super tuners using a chassis dyno
2. Again shows its mute point of adding larger airmass and paths for the heads only can flow so much
3. Even a 75 mm MAF can support the max volume flow of the runners.
4. With CNC process there should be zero reason these cyberspace vendors are selling heads for $2,000 plus.

Here is how a LS1 flow benches :



And here is LS6 heads:



Flow test shows that the LS6 intake manifold flowed substantially better than the LS1 manifold at all but low lifts. SuperFlow gives us an aproximate horsepower output per cylinder based on air flow measurements. With radius plate this figure is 66.8 HP per cylinder, with LS 6 manifold 60.4 HP per cylinder, and with LS1 manifold 55.6 HP per cylinder.
So according to SuperFlow's extrapolation the LS6 equipped engine should be able to produce 483.2 HP while the same engine with LS1 intake manifold will produce 444.8 HP or a loss of 38.4 HP.

If you look at the flow numbers per cylinder for the LS1 intake manifold you will note that the intake runner for cylinder #7 is not up to the other runners in flow. GM apparently fixed this deficiency on the LS6 intake manifold which showed much more even flow.

It has been shown time and again that steady state flow testing very closely approximates the results obtained by pulsating flow testing. SAE has several articles on the subject if you are interested in reading them.

Someone testing a custom head states :

Regarding the flow numbers. If the numbers look too good to be true they probably are. When I first took a look at the heads I knew there was no possible way they could flow much better than a stock set. No work done in the valve bowl area at all. Valves weren't even cut back. They were CNC "ported" but I bet almost nothing was removed from the runners.

The number to look at is the valve discharge coefficient. If this number is ridiculously high one knows right away there is something not right with the stated flow numbers. In essence a 2.02 valve is going to pass x amount of air per lift regardless of the port that is feeding it. Don't expect to see 320 - 330 cfm out of a 2.02 valve, it is not going to happen.

Another number you need to look at is the minimum port cross sectional area. Everyone likes to use port volume but that is not sufficient. Note that if we attach the intake manifold to the head the port volume now would be the volume from the intake runner opening to the valve. The longer the runner the greater that volume number is going to be. Not telling us anything about the area of the runner where it is smallest. That minimum cross sectional area in proportion to flow determines just how good the heads actually are and how much torque one can expect to get.

I have my opinions on intake to exhaust ratio related to power output as I am sure many others have that are in this business. I consider that proprietary information but I will say that a great deal depends on the camshaft timing and lift on the intake vs. exhaust valve. I have expensive simulation software to work this out but this is only applicable to high dollar engines where I can absorb the cost of running the simulations for hours on end.

Same is true of horsepower numbers as is true of flow numbers. I have access to five different engine dynos. The one that gives the lowest numbers is the one that gives the most accurate numbers by far.
When one sees very low brake fuel specific numbers, those approaching diesel engine numbers, one should see right away the horsepower numbers are too high. But horsepower numbers sell engines just like flow numbers sell heads.

Let the buyer beware!